Assuming your First Person Controller is named Player and related camera is named PlayerCamera, then there are two bugs in your script:
Function key names must contain lowercase f letter, so in your case you have to use f12 instead of F12. Instead of string names, I suggest using KeyCode enumeration.
Having the same condition for if and then for else if will cause that code inside else if will never be called. I suggest following solution:
#pragma strict
private var _playerMouseLook : MouseLook;
private var _playerCameraMouseLook : MouseLook;
function Awake() {
_playerMouseLook = GameObject.Find(“Player”).GetComponent(MouseLook);
_playerCameraMouseLook = GameObject.Find(“PlayerCamera”).GetComponent(MouseLook);
}
function Update() {
if(Input.GetKeyDown(KeyCode.F12)) {
enableCamera(!_playerMouseLook.enabled);
}
}